Universally, the windings of electromagnetic devices of comparatively large power input are cooled with fluids, such as oils or water, which remove the heat evolving in the windings due to ohmic losses. The mechanism of heat removal from the windings of such devices is based on either thermal convection or forced fluid flow. The latter approach has been used for cooling of electromagnetic stirrers, (abbreviated herein as EMS), widely used in metals processing industries. These stirrers are cooled by water supplied under pressure either from a dedicated source or for cooling of a casting mold.
In accordance with the most commonly used method, the cooling water flow fills in a space volume accommodating the stirring coils and extracts the heat from the outside of individual wires of the coil windings. FIGS. 1 and 2 show an embodiment of such a cooling system commonly used with an EMS for continuous casting of steel billets and blooms. The EMS 7 is arranged within a continuous casting mold assembly 1 which is comprised of a vertical mold 2 into which is received molten metal 4 which is surrounded by an EMS 7. The water flow 3 enters the windings 5 of the EMS at the bottom portion of the windings and travels upwardly in the space 8 provided between individual wires 9, then, as shown in FIG. 2, the flow 3 exits from the upper portion of the winding. With this cooling arrangement, the winding insulation is in direct contact with water. Because untreated water has rather high electrical conductivity, the water needs to be chemically treated to reduce the electrical conductivity to acceptable levels and/or the wire insulation is reinforced in order to eliminate any microscopic pores in the insulation to avoid a possibility of direct contact between the copper wire and water, which leads to copper erosion and eventual failure of the device. Moreover, both reliable wire insulation and a voltage limitation are required in order to prevent short circuiting between the rather tightly packaged windings, as the cooling water, even with a reduced electrical conductivity, is a poor insulating medium. In industrial practice, neither of the above approaches, i.e. the water electrical conductivity reduction or enhancement of electrical insulation, for example, with a resin, varnish or similar compounds, provides a guaranteed reliability of the stirring coils.
Another approach to cooling windings with water is to use a hollow conductor for winding manufacture. In hollow windings, the cooling water flows inside the conductor while the electrical insulation on the outside remains dry. The cooling water in that instance is also treated in order to avoid an electrolytic reaction causing deposits to be formed on the inside walls of tubular conductors. The above noted water-cooling systems for external or internal cooled windings comprise of a closed circuit water supply equipped with pumps, filters, instrumentation, etc. which adds to capital and operating costs of electromagnetic stirring systems.
A novel concept of cooling electromagnetic devices with fluids which display magnetic behaviour became known in the 1960's (ref. R. E. Rosensweig, Ferrohydrodynamics, Cambridge University Press, 1985). An interaction between magnetic fields and magnetic fluids results in a body force which sets the fluid in motion. This property of magnetic response is used in many practical applications, including cooling of electromagnetic devices.
U.S. Pat. No. 5,898,353 describes the use of a magnetic fluid for convective cooling of a distribution transformer. A gradient of magnetic field produced by the transformer produces a circulation pattern in magnetic fluid which cools the transformer windings submerged in the fluid.
U.S. Pat. No. 5,863,455 describes methods of cooling electromagnetic devices, including power transfomers, with magnetic colloidal fluid which has improved insulating and cooling properties. The patent refers to an electromagnetic device comprising means for producing an electromagnetic field, heat, and a stable colloidal insulating fluid which is in contact with the device. The magnetic fluid in the above application has a saturation magnetization of about 1 to 20 Gauss. An electromagnetic device relevant to that patent was a power transformer.
Other prior art includes U.S. Pat. Nos. 4,506,895, 4,992,190 and 5,462,685.
In spite of these prior art disclosures, electromagnetic stirrers employed in the metals processing industries, and continuous casting of steel in particular, remain water-cooled, except for stirrers with a very limited power input which may be air-cooled. The water-cooled systems impose special requirements and equipment for treatment of water, instrumentation for monitoring and maintaining its properties, special demand for electrical insulation integrity, special equipment (e.g. pumps, filters, piping, etc.) which makes reliability and performance of the stirrers dependent on the above parameters and equipment. This dependence can be, and often is compromised by defects in stirrer fabrication, materials used, equipment malfunction or human error.